1. Field of the Invention
The present invention relates to a method for forming a trench type element isolation structure to be used for a semiconductor integrated circuit and trench type element isolation structure.
2. Description of the Prior Art
In a semiconductor integrated circuit, in order to eliminate electric interference between the elements in operation and control each element in fully independent state, elements are isolated from one another. Especially, a trench type element isolation trench is a structure filled with an insulator, and, as a bird""s beak does not occur, it is an indispensable element isolation structure for making the semiconductor integrated circuit into miniature size.
FIGS. 13A-13F are sectional views of a method of forming a conventional trench type element isolation structure. First, as shown in FIG. 13A, an under-layer oxide film (first thermal oxidation film) 2 and a silicon nitride film 3 are sequentially deposited on a silicon substrate 1, after which, with a photolithographic pattern (not illustrated) used as a mask, silicon nitride film 3 and under-layer oxide film 2 are sequentially patterned to form a groove in silicon substrate 1.
Next, as shown in FIG. 13B, a thermal oxidation film 10 is formed on an inner wall of the groove by thermal oxidation, after which an imbedding oxide film 11 is laid over the whole surface by CVD step.
Next, as shown in FIG. 13C, the imbedded oxide film 11 formed on the upper part of the nitride film 3 by a CMP step using a silicon nitride film 3 as a stopper is eliminated to allow the imbedded oxide film 11 to remain only in the groove.
Next, as shown in FIG. 13D, the silicon nitride film 3 is eliminated by heated phosphoric acid, after which the CVD oxide film 20 is accumulated on the whole surface by CVD step.
Next, as shown in FIG. 13E, a CVD oxide film 20xe2x80x2 is allowed to remain only on the side wall of the imbedded oxide film 11 by carrying out anisotropic etching.
Finally, as shown in FIG. 13F, by eliminating the under-layer oxide film 2 with hydrofluoric acid, a trench type element isolation structure is completed.
In a method of forming a trench type element isolation structure, it is essential to remove ultimately the under-laid oxide film 2 formed on the activated region 23. However, in the conventional structure trench type element isolation structure, the CVD oxide film 20xe2x80x2 is formed by CVD and the etching speed in hydrofluoric acid is larger than that of the thermal oxidation film therefore in removing under-laid oxide film 2 shown in FIG. 13F, the CVD oxide film 20xe2x80x2 is also etched and, hence, fails perform the function as protective film for the oxide film 11 imbedded in the groove. Thus the imbedded oxide film 11 in the groove is also etched in the edge part, resulting in formation of a recess 21 on the edge part of the imbedded oxide film in the groove.
In an integrated circuit, as shown in FIG. 16, there may be a case where a gate electrode 22 is formed on said trench type element isolation to take a structure to control the transistor formed on the activated region 23 by said gate electrode 22. In such a case, due to the existence of the recess 21, the gate electrode 22 does not become smooth shape on the edge part of the trench but concentration of an electric field occurs, which may be a cause for the reverse narrow channel effect to show lowering of the threshold value of the transistor. Especially, as the integration of the semiconductor elements progresses and the width of the activated region 23 (gap between the adjacent trenches) becomes narrower, the effect of the reverse narrow channel effect becomes remarkable, thereby making it extremely difficult to control the threshold voltage of the transistor to give ill effect on the circuit operation.
Accordingly, the present invention has its object to provide a method for forming a trench type element isolation structure which is free from formation of recess in an edge part of a trench type element isolation imbedded oxide film.
In view of the above, the present inventors made strenuous study, and as a result found out that, by forming a thermal oxidation film having higher etching resistance than the CVD film not only on the surrounding of the imbedded oxide film inside the groove formed on the silicon substrate but also on the lateral side of the imbedded oxide film projecting upward from the silicon substrate, formation of recess on the edge part of the imbedded oxide film in the step of removing the first thermal oxidation film can be prevented, and the reverse narrow channel effect of the transistor wherein a gate electrode is formed on a trench type element isolation structure can be suppressed, and completed the present invention.
According to the present invention, there is provided a method for forming a trench type element isolation structure wherein an imbedded oxide film projecting upward from the silicon substrate surface is imbedded in a groove formed on a silicon substrate through a thermal oxidation film, comprising; a) a step of forming a non-single crystal silicon film on said silicon substrate through a first thermal oxidation film, b) a step of forming a groove extending from the surface of said non-single crystal silicon film to the inside of said silicon substrate, c) a thermal oxidation step for forming second and third thermal oxidation films on the inside surface of said groove including said groove wall and said lateral side wall of said non-single crystal silicon film, and d) a removing step for removing said non-single crystal silicon film excluding said third thermal oxidation film to have said third thermal oxidation film formed on the lateral side projecting upward from the surface of said silicon substrate of said imbedded oxide film.
According to such a method, because the surroundings of the imbedded oxide film, not only the silicon substrate surface inside the groove but also the lateral side of the upward projecting part from the silicon substrate surface, are surrounded by the thermal oxidation film having the higher etching resistance than the CVD oxide film, such thermal oxidation film is less liable to be etched in the etching step of the first thermal oxidation film.
That is to say, in the conventional method, the CVD oxide film formed on the surroundings of the imbedded oxide film projecting upward from the silicon substrate surface showed formation of recess at the edge part of the imbedded oxide film inside the groove, by being simultaneously etched in the etching step of the first thermal oxidation film. To the contrary, according to the present invention method, on the lateral side of the imbedded oxide film projecting upward from the silicon substrate surface is provided with a thermal oxidation film having higher etching resistance than the CVD oxide film, so that the film is less liable to be etched in the etching step of said first thermal oxidation film, the lateral side of the imbedded oxide film is protected, and it becomes possible to prevent formation of recess in the edge part of the imbedded oxide film.
Thus, in the transistor having a gate electrode formed on a trench type element isolation structure, because of no formation of recess on the imbedded oxide film inside the isolation groove, concentration of electric fields in the gate electrode formed on the imbedded oxide film as in the conventional instance can be prevented, and it becomes possible to suppress the reverse narrow channel effect of transistor.
The present invention also provides a method for forming a trench type element isolation structure which comprises, further between said thermal oxidation step and said removing step, a deposition step for depositing said imbedded oxide film inside said groove and on said non-single crystal silicon film, and a film-thinning step for reducing the film thickness from the upper surface of the imbedded oxide film until the non-single crystal silicon film is exposed, characterized by forming said third thermal oxidation film on the projecting side wall projecting upward from the surface of the silicon substrate.
Especially, according to the present forming method, unlike the conventional method, a step of removing the film by dry etching is not included, and it becomes possible to prevent occurrence of damage in the substrate.
The present invention is also a method for forming a trench type element isolation structure which comprises, a step for forming silicon nitride films on said non-single crystal silicon films on both sides of said groove, and further comprises, in said film-thinning step, a step of removing said silicon nitride film after reducing the film thickness of said imbedded oxide film with said silicon nitride film used as a stopper and leaving said imbedded oxide film only inside the groove.
According to such a forming method, in the etching step of the first thermal oxidation film, in addition to it being possible to prevent formation of recess in the edge part of the imbedded oxide film in the trench, due to the use of a silicone nitride film which has a large etching selectivity to the silicon oxide film as a stopper in the film thinning step, it is possible to control the height of the imbedded oxide film from the substrate surface in good precision and to reduce irregularity of the heights of the imbedded oxide film.
The present invention is also a method for forming a trench type element isolation structure which comprises, further between the deposition step and the film-thinning step, a step for removing said imbedded oxide film, the thermal oxidation film on the upper surface of said non-single crystal silicon film, and said third thermal oxidation film, from the upper part to the predetermined position underneath the upper surface of the non-single crystal silicon film, and a step for depositing the upper-layer imbedded oxide film on said groove inside and said non-single crystal silicon film.
According to such forming method, in the etching step of the first thermal oxide film, in addition to it being possible to prevent formation of recess in the edge part of the imbedded oxide film in the trench, in the deposition step, in case of formation of a seam in the imbedded oxide film, the imbedded oxide film is eliminated until such seam is exposed from the upper part, and the upper layer imbedded oxide film is deposited so as to bury such seam, by which it becomes possible to form a seamless element isolation structure.
As a result, in case of forming an electrode on the trench type element isolation, short-circuiting of electrode attributed to said seam is prevented, and improvement of yield in manufacture of the integrated circuit can be expected.
The present invention is also a method for forming a trench type element isolation structure which comprises, further between the deposition step and the film-thinning step, a step for removing said imbedded oxide film, the thermal oxidation film on the upper surface of said non-single crystal silicon film, and said third thermal oxidation film, from the upper part to the predetermined position between the upper surface and the bottom surface of the non-single crystal silicon film, to expose a side surface of at least a part of said non-single crystal silicon film, a step for thickening the tip parts of the third thermal oxidation films on both sides of said imbedded oxide film by thermally oxidizing the exposed side and the upper surface of said non-single crystal silicon film, and a step of depositing the upper layer imbedded oxide film on the whole surface after thickening the tip part of said third thermal oxidation film.
According to such forming method, the third thermal oxidation film of the part A of the side wall part shown in FIG. 4E can be formed thicker than the other thermal oxidation film, and in the etching step of the first thermal oxidation film, the third thermal oxidation film on the A part is less easily etched, so that the formation of recess on the edge part of the imbedded oxide film can be more effectively prevented.
Since the thickness of the third thermal oxidation film on the part A may be individually formed thick without thickening the film thickness of the second thermal oxidation film in inside of other trench groove, even in case of using the forming method of the present invention, the film thickness of the second thermal oxidation film in the trench groove does not become thick, and there is no generation of seam due to the increase in the aspect ratio.
Further, the present invention is also a method for forming a trench type element isolation structure which comprises, further between said film-thinning step and said removing step, a step for thickening the upper end part of said third thermal oxidation film by thermal oxidation of the exposed upper surface of said non-single crystal silicon film, and a step for removing the thermal oxidation film on the upper surface of said non-single crystal silicon film, after thickening the upper end part of said third thermal oxidation film.
According to such forming method, the third thermal oxidation film of the part B shown in FIG. 5E can be formed thicker than the other thermal oxidation film, and in the etching step of the first thermal oxidation film, the third thermal oxidation film on the part B is less easily etched, so that the formation of recess on the edge part of the imbedded oxide film can be more effectively prevented.
The present invention is also a method for forming a trench type element isolation structure which comprises, further between the deposition step and the film thinning-step, a step for removing said imbedded oxide film, the thermal oxidation film on the upper surface on said non-single crystal silicon film and said third thermal oxidation film from the upper part to the predetermined position between the upper surface and the bottom surface of the non-single crystal silicon film, to expose a side surface and the upper surface of the non-single crystal silicon film, a step for depositing the non-single crystal silicon film for thermal oxidation on the whole surface, a step for thermal oxidation of said non-single crystal silicon film for thermal oxidation, and a step for depositing the upper layer imbedded oxide film on the whole surface.
According to such forming method, a thermal oxidation film having high etching resistance can be formed on the upper part in addition to the side wall of the imbedded oxide film in the groove. In other words, because the imbedded oxide film inside the groove is surrounded by the third thermal oxidation film even on the upper surface, so that the formation of recess on the edge part of the imbedded oxide film in the etching step of the first thermal oxidation film can be perfectly prevented.
Further, as it is possible to form the third thermal oxidation film of part C in FIG. 6F thicker than other thermal oxidation film in the groove, it becomes possible to expect strengthening the part which has so far been most liable to be etched in the etching step of the first thermal oxidation film.
Besides, in case a seam has been formed in the imbedded oxide film, the imbedded oxide film is removed from the upper part to a degree that such seam does not remain, after which the non-single crystal silicon film for thermal oxidation is deposited over the whole surface, by which it becomes possible to form a seamless element isolation structure.
The present invention is also a method for forming a trench type element isolation structure, which comprises, a step for depositing an imbedded oxide film on said groove inside and said non-single crystal silicon film, before said thermal oxidation step, and further, said thermal oxidation step is a step for forming a said second and third thermal oxidation film by carrying out thermal oxidation over said imbedded oxide film.
According to such forming method, in addition to the fact that the formation of recess on the edge part of the imbedded oxide film in the trench, in the etching step of the first thermal oxidation film can be prevented, because the thermal oxidation step is operated after depositing the imbedded oxide film, the imbedded oxide film is kept at a high temperature in the thermal oxidation step, or so-called bake-tightened, and especially, as in the part shown as 40 in FIG. 17, it becomes possible to reinforce the joint of the weak joint part 40 of the imbedded oxide film.
Accordingly, as shown in FIGS. 17A-C, it becomes possible to prevent formation of a recess in the imbedded oxide film which has been experienced in the conventional method, and to form an element isolation structure having good isolation characteristics.
The present invention is also a method for forming a trench type element isolation structure which comprises, further a film-thinning step for reducing the film thickness from the upper surface of the imbedded oxide film until said non-single crystal silicon film expose, between the thermal oxidation step and said removing step, and characterized by forming said imbedded oxide film so as to form said third thermal oxidation film on the side surface of said imbedded oxide film projecting upward from the surface of said silicon substrate.
Especially, according to this forming method, because there is not included a film removing step by dry etching as in the conventional step, it is possible to prevent formation of damage on the substrate.
The present invention is also a method for forming a trench type element isolation structure which comprises, further a step for forming a silicon nitride film on said non-crystal silicon film on both sides of said groove, and further comprises, in said film-thinning step, steps for reducing the film thickness of said imbedded oxide film using said silicon nitride film as a stopper, leaving said imbedded oxide film only in said groove, followed by removing said silicon nitride film.
According to such forming method, due to the use of a silicon nitride film having large etching selectivity to the silicon oxide film as a stopper in the film thinning step, it is possible to control the height of the imbedded oxide film from the substrate surface in good precision, and to lessen the dispersion of the heights of the imbedded oxide film from the substrate surface.
The present invention is also a method for forming a trench type element isolation structure which comprises, a step for reducing the film thickness from the upper surface of the imbedded oxide film until said non-crystal silicon film is exposed, between said deposition step and said thermal oxidation step, and further, a step for removing said thermal oxidation film from the upper surface of the non-single crystal silicon film, between said thermal oxidation step and said removing step.
Especially, according to such forming method, the oxidation rate of the lateral side wall of the non-single crystal silicon film which has a short distance for the oxidation species to diffuse through the imbedded oxide film is larger than the oxidation rate of the groove bottom part and the like, and accordingly, it becomes possible to form the film thickness of the third thermal oxidation film formed on the lateral wall part thicker than the film thickness of the second thermal oxidation film formed on the groove bottom part.
The present invention is also a method for forming a trench type element isolation structure which comprises, a step for forming a silicon nitride film on said non-single crystal silicon films on both sides of said groove, and further a step for reducing the film thickness of said imbedded oxide film using said silicon nitride film as a stopper and leaving said imbedded oxide film only inside the groove, between said deposition step and said thermal oxidation step, and further, a step for removing said silicon nitride film after the thermal oxidation step.
Even according to such a method, the height of the imbedded oxide film from the substrate surface can be controlled in good precision, and the dispersion of the heights of the imbedded oxide film can be made small.
The above thermal oxidation step is preferably a high temperature thermal oxidation step to be performed at 1000xc2x0 C. or higher.
It is because, by carrying out the thermal oxidation step at 1000xc2x0 C. or higher, it becomes possible to tighten by baking the previously deposited imbedded oxide film, and to strengthen the bonding of the weak joint of the imbedded oxide film.
The above forming method may include a thermal oxidation step for oxidizing said surface of said groove wall and on the side wall part of said non-single crystal silicon film, between said groove formation step and said thermal oxidation step.
By forming a thermal oxidation film twice, particularly it becomes possible to form the film thickness of the second thermal oxidation film inside the grooves to be formed in layers and the third thermal oxidation film on the side wall part of non-single crystal silicon film in thick layers.
The film thickness of said third thermal oxidation film is preferably 30-100 nm, because the film is not etched in the step for removing the first thermal oxide film.
The film thickness of said third thermal oxidation film is preferably thicker than that of the second thermal oxidation film.
The present invention also provides a trench type element isolation structure comprising an imbedded oxide film projecting upward from said silicon substrate surface being imbedded in a groove formed on a silicon substrate through a thermal oxidation film, characterized in that the film thickness of the third thermal oxidation film formed on the surface of said groove higher than said silicon substrate surface and the second thermal oxidation film formed on the surface of said groove lower than said silicon substrate surface has an overhang part gradually overhanging outward in the vicinity of said silicon substrate surface so that the film thickness of said second and third thermal oxidation film in a direction perpendicular to the groove wall of said groove becomes the thickest in the vicinity of said silicon substrate surface.
According to such trench type element isolation structure, unlike the conventional structure, no recess is formed on the edge part of the imbedded oxide film and the upper part is flat, and further, the third thermal oxidation film smoothly overhangs outward in the vicinity of the substrate surface. Accordingly, in case of forming a transistor having a gate electrode on a trench type element isolation structure, the gate electrode can be formed with a smooth shape especially at the bottom part, so that it becomes possible to prevent occurrence of electric field concentration in the gate electrode at the upper part of the edge of the imbedded oxide film which has been experienced with the conventional structure, and to prevent reverse narrow channel effect of the transistor.
The above film thickness of said third thermal oxidation film is preferably thicker than that of the second thermal oxidation film.
It is preferable for the upper surface of said imbedded oxide film to be further covered with a thermal oxidation film.
The present invention also provides a DRAM memory cell structure using a trench type element isolation structure, comprising a plurality of MOS transistors formed on a semiconductor substrate, an insulation film formed on said MOS transistor, and a bit line and a capacitor electrode which are respectively electrically connected with the source region and drain region of the MOS transistor through a contact hole provided on said insulation film, wherein electrical insulation is made between said plural MOS transistors by the trench type element isolation structure formed on said semiconductor substrate.
As described above, by using a trench type isolation structure of the present invention, development of a reverse narrow effect in the integrated memory cell can be prevented, and dispersion of inter-memory cell element characteristics attributed to the dispersion of channel widths can be suppressed, thereby making it possible to obtain stabilized operation of the DRAM memory cell.